Conventionally, for generally known discharge lamp lighting device for electrostatic copying machines, there has been one, as shown, for example, in FIG. 1, which is started by an application of a secondary voltage (for example, of 150 volts) of a leakage autotransformer T.sub.1 to a filament preheating type fluorescent lamp L.sub.1 which is a light source through a phase advancing capacitor C.sub.1 by closing a switch S.sub.1. However, according to such conventonal type, there is a possibility that, as its discharge circuit forms a phase advancing circuit, the light output wave form will be disturbed and the period in which the light output of the fluorescent lamp exceeds a required level L will be so short that, in the period in which the light output is not sufficient, the copying will not be made normal (see FIG. 2A). More specifically, in the period in which the light is not sufficiently provided, the irradiated amount of light on the object to be copied will be so small that, even if the object is white, it will be discriminated to be black and no fidelity copying will be made any more.
Such problem is not limited only to the above described typical conventional lighting device but also exists in the case of obtaining a light output wave form substantially of a sine wave form by lighting the discharge lamp with a use of, for example, a choke coil type stabilizer. That is, in this case, too, the period in which the light output of the fluorescent lamp exceeds the required level L will be so short that there will be the same problem as described above (see FIG. 2B) and, in these cases, an elevation of the absolute value of light amount of the discharge lamp to some extent does not result in a fundamental solution of the problem.
Referring again to the above described problem in the discharge lamp lighting circuit of FIG. 1 shown as a typical conventional example, a lamp current of about 0.8 A. flows through the fluorescent lamp L.sub.1 after it is lighted, the lamp voltage of the fluorescent lamp in this case is about 40 volts, so that the ratio of the secondary voltage V.sub.2 of the transformer required for starting this lamp to the lamp voltage V.sub.La will be V.sub.2 /V.sub.La .apprxeq.3.8, wherein.apprxeq.means "substantially equals". Thus, the value of V.sub.2 /V.sub.La in the general discharge lamp lighting devices for copying machines shows a value generally considerably larger than the value of about 2.0 of general illumination stabilizers for starting general illumination lamps which are improved for easy starting. Here, the stabilizers generally high in this value of V.sub.2 /V.sub.La are preferable since fluctuations in the lamp current with respect to fluctuations in the source voltage are low but, on the other hand, in the case of forming a phase advancing circuit, the wave form of the lamp current will deteriorate so that the light output wave form of the lamp will also deteriorate. As has been described above, in the case of the stabilizer of the mere choke coil type as shown in FIG. 2B, a lamp current substantially of a sine wave form can be obtained, whereas, in such stabilizer of the phase advancing type as in the above described typical conventional example, time sections in which substantially no light is emitted will exist during about one half of the current source voltage. When the frequence of such non-light emitting time is represented by an expression of form factor (=effective value/mean value):
Form factor f of single choke coil type stabilizer . . . f.apprxeq.1.12 PA1 Form factor f of phase advancing type stabilizer (shown in FIG. 1) . . . f.apprxeq.1.20 PA1 (a) Direct current lighting, PA1 (b) High frequency lighting, and PA1 (c) Flickerless circuit formation.
Thus, it is understood that, even if the effective value of the lamp current is 0.8 A. in each, the light amount in the latter case will be smaller by more than several % than in the case of the former and much more sections of non-light emittion exist. It should be additionally referred to that, in the former case, the light output is satisfactory at the minimum but the fluctuation characteristics of the current source voltage are poor and involve certain problem in practice.
Now, in order to accelerate an understanding of the present invention to be disclosed later, references shall be made here to the characteristics of the fluorescent lamp which has been used as the light source of the electrostatic copying machines of the kind referred to. In the most general fluorescent lamp (FLR 30) for the copying machines, a predetermined light output is obtained by flowing a lamp current (0.8 A.) which is about 1.5 times as large as that of such general illumination fluorescent lamp as, for example, of FL 30 (of a lamp current of 0.6 A.), so that a difference will naturally exist in respect to the ambient temperature characteristics from the general illumination fluorescent lamps. This difference is shown in FIG. 3 in which, in the case of FL 30 as an example of the general illumination fluorescent lamps, the maximum flux of light is provided as shown by the solid line in the drawing when the ambient temperature is about 20.degree. C. but, on the other hand, in the case of FLR 30 as an example of the flourescent lamps for the copying machines, the value of the maximum flux of light is present adjacent 5.degree. C. as shown by the broken line in the drawing and, at a temperature higher than it, the flux of light steadily reduces. This is due to that not only the lamp current is large as described above but also the tube diameter of the fluorescent lamp is 25.phi. for the copying machines while the tube diameter of the general fluorescent lamps is 38.phi. so that the current density will be three times as high as that of the general ones, whereby the interior of the copying machines is made unexpectedly high in the temperature and the ambient temperature of the lamp often reaches 80.degree. C. Even if it is attempted, therefore, to increase the light output by further increasing the current in anticipation of it, no sufficient light output will be obtained. These circumstances are shown in FIG. 4, that is, as shown by the solid line in the drawing, the light output increases with the increase in the current in the general fluorescent lamps but, in the fluorescent lamps for the copying machines, any increase in the light output will be canceled by reductions in the lamp efficiency so as to be substantially constant even if the current is increased, as shown by the broken line, so that no increase can be expected at all in the light output even with the increase of the current. It may be possible to consider an improvement of the current density in order to increase the light output of the fluorescent lamp for the copying machines under such circumstances. As a measure for performing such improvement, however, a selection must be made either from an enlargement of the tube diameter of the lamp or a reduction of the lamp current, but either one can hardly be adopted since, with the former, the device must become larger in size and, with the latter, the absolute value of the light amount will be reduced.
In the general conventional discharge lamp lighting devices for copying machines, as has been disclosed in the foregoings, there have been defects that, though it is set to flow a larger current to the lamp than in the general illumination fluorescent lamp lighting devices in order to obtain a larger light amount, the high temperature characteristics are not preferable due to the relation to the tube diameter of the lamp and, in the case of using a stabilizer in which a phase advancing discharge circuit compensating for fluctuations in the current source voltage is formed, the ratio of V.sub.2 /V.sub.La will be generally large due to the relation to the lamp starting voltage and many non-light emitting sections will exist in the lamp current wave form. In the device of FIG. 1 shown as a typical conventional system, it is seen that the current wave form will be as shown in FIG. 2A so that the peak value of the lamp current will become extremely high and the lamp life will be made short, and further that this large current point will be short in the time, substantially no current will contribute to the copying function and the current is flowed wastefully.
Further, it is of course possible to employ such quite distinctive lighting systems as follows in order to avoid these defects of the exemplified conventional devices:
In the system of (a), first, there is a difficulty that a difference in the brightness exists between the cathode and anode of the lamp. That is, the difficulty is present due to a Faraday dark space existing on the cathode side to render the brightness different between the cathode and anode, whereby the entire length of the light source can not be utilized. In the system of (b), next, the lamp is lighted by high frequency waves so that there will be no non-light emitting section of the lamp but the device is costly. In the system of (c), finally, two lamps must be provided to be lighted alternately, so that the cost becomes high and, further, there is a difficulty in the fluctuation characteristics of the current source voltage on the phase delaying side fluorescent lamp. The present invention is not to solve such technical problems as have been disclosed by means of any system different from such generally adopted conventional systems as these, but is to dissolve the various problems by improving the above described general conventional systems.